Lowering the temperature of dividing endosperm of Haemanthus (H. katherinae and H. albiflos) from 24 degree C to 0 degrees C (or even to plus 3.5 degrees C) results in the arrest of chromosome movements within 2-4 minutes. These movements can be reinitiated within 2-5 minutes by returning cells to room temperature, even if the low temperature treatment has been extended for two hours. Low temperature affects both the number and the arrangement of microtubules (MTs). After exposing cells for 10 minutes at plus 2 degrees C, the total number of spindle MTs is reduced to 1/3-1.4 that of anaphase controls. Some kinetochore MTs persist in the temperature range plus 6.5 degrees C to plus 1 degrees C. The arrangement of the persisting kinetochore MTs changes during the treatment from a "fanned out" configuration (control) to a parallel one (experimental). These parallel kinetochore fibers of cells arrested at early anaphase contain more MTs than kinetochore fibers of mid to late anaphase controls in which chromosomes are still moving. Some of these parallel MTs are at least 20 micron n long (early anaphase) and they disassemble slowly during prolonged exposure to low temperature (plus 2 degree C or below). After returning temperature-shocked cells to room temperature, the number of spindle MTs increases to a level one-third higher than control cells in a comparable stage of division. In addition, the arrangement of MTs reverses from a parallel configuration to fanned configuration and chromosome movement is reinitiated. Both of these processes are complete within 10 minutes after return to ambient temperature. The data acquired from temperature experiments have several important theoretical implications which are discussed in detail.